adapt for column collection being "immutable" or "readonly"
adapt for column collection being "immutable" or "readonly"

in 2.0 the ColumnCollectionConstraint will use a
"read only" column collection, renamed from "immutable".
Add logic to detect both of these kinds of collections

Change-Id: I4049c3081a2b3801ea7bfa31bb62bafb3eae4a9d

sqlalchemy/alembic,zzzeek/alembic